Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

[H. Peter Anvin]

On 04/24/2014 03:31 PM, Andrew Lutomirski wrote:
> 
> I was imagining just randomizing a couple of high bits so the whole
> espfix area moves as a unit.
> 
>> We could XOR with a random constant with no penalty at all.  Only
>> problem is that this happens early, so the entropy system is not yet
>> available.  Fine if we have RDRAND, but...
> 
> How many people have SMAP and not RDRAND?  I think this is a complete
> nonissue for non-SMAP systems.
> 

Most likely none, unless some "clever" virtualizer turns off RDRAND out
of spite.

>>> Peter, is this idea completely nuts?  The only exceptions that can
>>> happen there are NMI, MCE, #DB, #SS, and #GP.  The first four use IST,
>>> so they won't double-fault.
>>
>> It is completely nuts, but sometimes completely nuts is actually useful.
>>  It is more complexity, to be sure, but it doesn't seem completely out
>> of the realm of reason, and avoids having to unwind the ministack except
>> in the normally-fatal #DF handler.  #DFs are documented as not
>> recoverable, but we might be able to do something here.
>>
>> The only real disadvantage I see is the need for more bookkeeping
>> metadata.  Basically the bitmask in espfix_64.c now needs to turn into
>> an array, plus we need a second percpu variable.  Given that if
>> CONFIG_NR_CPUS=8192 the array has 128 entries I think we can survive that.
> 
> Doing something in #DF needs percpu data?  What am I missing?

You need the second percpu variable in the espfix setup code so you have
both the write address and the target rsp (read address).

	-hpa


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